
for (i=0; i<N; i++) {
  for (j=0; j<N; j++) {
    x1[i] = x1[i] + a[i][j] * y_1[j];
  }
 }

for (i=0; i<N; i++) {
  for (j=0; j<N; j++) {
    x2[i] = x2[i] + a[j][i] * y_2[j];
  }
 }


